The invention relates to a multi-layer sliding bearing comprising a back metal layer, a bearing alloy layer, an intermediate plating layer and a Sn-based overlay layer.
Hitherto, there are known alloy bearings each provided with an overlay layer for enhancing initial conformability and foreign matter embeddability, and regarding the overlay layer, for example, Sn-based or Pb-based overlay layer for an aluminum alloy sliding bearing are known. In general, when forming the overlay layer on the Al bearing alloy layer, wet type plating is usually adopted. However, in a case where a plating layer of Sn based or Pb based alloy is formed directly on the Al bearing alloy layer, the resultant bonding strength between them becomes inferior. To prevent this unfavorable inferior bonding strength from occurring, there is provided an intermediate plating layer of a hard metal such as Ni or Fe or Co, on which intermediate plating layer an overlay layer having a high bonding strength strength is formed by the plating of the Sn-based or Pb-based alloy.
In the conventional Al alloy sliding bearing provided with the intermediate plating layer, the hard intermediate plating layer is exposed when the overlay layer is worn out during the use of the sliding bearing, so that the seizure and/or scuffing of the bearing becomes apt to occur. To cope with the problem, it is attempted to obtain a sliding bearing without any intermediate plating layer. For example, in JP-A-1-307512 there is disclosed a sliding bearing in which an Al bearing alloy layer is covered with a Sn-based overlay layer. In this prior art, there is formed, between the Al bearing alloy layer and the overlay layer, a thin intermediate layer not more than 0.5 xcexcm in thickness in which any one of Ni, Co and Fe exists in a mixture state with the components of the Sn-based overlay layer, whereby the bonding strength becomes large between the Al bearing alloy layer and the overlay layer while preventing the seizure and/or scuffing from occurring when the overlay layer is worn out.
The anti-seizure property of the Sn-based overlay layer is inferior to that of the Pb-based overlay layer. Thus, in the sliding bearing disclosed in JP-A-11-132235, the overlay layer is made of a Pbxe2x80x94Sn alloy so that the anti-seizure property thereof may be improved. In this prior art, the overlay layer of the Pbxe2x80x94Sn alloy is formed by a method comprising the steps of: performing in this order a zincate treatment on an Al bearing alloy layer, a thin film-plating treatment for forming Ni or Co or Fe layer of 0.05 to 0.5 xcexcm in thickness, a Sn-plating treatment for forming a Sn layer of 1 to 3 xcexcm in thickness, and a Pb-plating treatment for forming a Pb or Pb alloy layer; and then performing a diffusion treatment by heating the layers in order for Sn in the Sn layer to be diffused so that both of an overlay layer of Pbxe2x80x94Sn alloy and a bonding strength layer in which both of Sn and any one of Ni, Co and Fe exist in the mixture state may be formed.
As explained above, in the sliding bearing of JP-A-11-132235, the overlay layer of Pb based alloy is used for improving the anti-seizure property. However, since Pb causes bad influence in the environment, it is preferred to reduce Pb as little as possible and, if possible, to avoid the use of Pb. Further, in the overlay layer of the Pb-based alloy there is another problem that the overlay layer of the Pb-based alloy reacts with Sn in the Sn plating layer to thereby form a eutectic structure of a low melting point, which eutectic structure causes layer-like wear leading to the exfoliation of the overlay layer.
The invention is achieved in view of the above-explained situation, and the object of the invention is to obtain a multi-layer sliding bearing causing no substantial problem regarding anti-seizure property while reducing the amount of Pb additive or, if possible, while using no Pb, and at the same time having superior wear resistance.
In order to achieve the objects mentioned above, according to a first aspect of the invention, there is provided a multi-layer sliding bearing comprising a back metal layer, a bearing alloy layer, an intermediate plating layer and a Sn-based overlay layer, the Sn-based overlay layer containing reinforcing metal and/or inorganic particles, the content of the reinforcing metal and/or inorganic particles being varied step-wise or successively in the direction of thickness of the overlay layer so that the content of the reinforcing metal and/or inorganic particles contained in a thickness-wise center part of the overlay layer being relatively large and so that the content of the reinforcing metal and/or inorganic particles contained in the surface part (, that is, in the outermost part) of the overlay layer being zero or smaller than the content in the center part.
In the boundary portion between the Sn-based overlay layer and the intermediate plating layer, since both of Sn and metals constituting the intermediate plating layer exist in a mixed state, the bonding strength of the Sn-based overlay layer is enhanced. Further, since in the overlay layer no Pb is contained, there is no problem of the occurrence of the layer-like wear between the Sn-based overlay layer and the intermediate plating layer.
Further, on the surface of the Sn-based overlay layer there is contained no reinforcing metal or inorganic particles, or the amount thereof is very small if it is contained. Thus, because of the relatively soft Sn, it becomes possible to obtain good initial conformability and foreign matter embeddability. In addition, since in the center of the Sn-based overlay layer the reinforcing metal and/or inorganic particles are contained therein, an alloy of Sn and the reinforcing metal is formed, or the inorganic particles are distributed in the Sn or Sn alloy. According to the constitution, the mechanical strength of the Sn-based overlay layer is enhanced, the wear resistance thereof becoming high, and the anti-seizure property thereof is improved. Of course, since no Pb is contained in the Sn-based overlay layer, the sliding bearing of the invention is superior in view of environmental protection. Incidentally, the term xe2x80x9ccenterxe2x80x9d of the Sn-based overlay layer means not only the literal xe2x80x9cdimensional centerxe2x80x9d thereof but also a center portion of a somewhat broad range.
In this case, according to a second aspect of the invention, the bearing alloy layer may be made of a Cu alloy containing no Pb or an Al alloy containing no Pb, in view of environmental protection.
Further, according to a third aspect of the invention, the reinforcing metal contained in the Sn-based overlay layer may be at least one kind selected from the group consisting of Cu, Sb, Ag, In, Zn, Ni, Co, and Fe, and the Sn-based overlay layer is made of a Sn-based alloy containing one or more kinds of reinforcing metal of 0 to 30 mass % and/or inorganic particles of 0 to 25 volume %.
The at least one kind selected from the group consisting of Cu, Sb, Ag, In, Zn, Ni, Co, and Fe forms an alloy with Sn of the Sn-based overlay layer to thereby enhance the mechanical strength and wear resistance of the overlay layer. Further, the inorganic particles is distributed in Sn or Sn alloy of the Sn-based overlay layer to thereby enhance the mechanical strength and wear resistance of the Sn-based overlay layer. In a case where the amount of the reinforcing metal exceeds 30 mass % or the amount of the inorganic substance exceeds 25 volume %, the Sn-based overlay layer becomes excessively hard in hardness with the result that the resistance to fatigue is deteriorated and/or that the attacking property thereof to a counterpart becomes large unfavorably.
According to a fourth aspect of the invention, the intermediate plating layer having a thickness of 0.01 to 8 xcexcm may be made of one metal selected from the group consisting of Ni, Co, Fe, Cu, and Ag or is made of an alloy containing no Pb and containing as the main constituent thereof this metal.
The intermediate plating layer acts to improve the bonding strength of the Sn-based overlay layer and to prevent the components of the Sn-based overlay layer from being diffused into the bearing alloy layer. The metal selected from the group consisting of Ni, Co, Fe, Cu, and Ag has a remarkable bonding strength property and superior effect of preventing the diffusion. However, in a case where the thickness of the intermediate plating layer is less than 0.01 xcexcm, the advantageous effect thereof is lowered, and in another case where this thickness exceeds 8 xcexcm, the cost of the material becomes large in comparison with the degree of the improvement.
According to a fifth aspect of the invention, a back plating layer may be provided on the back of the back metal layer.
For example, in an engine for an automobile, the lowering of rigidity regarding a bearing housing is caused due to a small-shape and light-weight design thereof, so that a fretting phenomenon (, that is, a wear phenomenon occurring between two contact faces subjected to relative movement of micro-amplitude) on the back of a sliding bearing becomes a serious problem. This problem of the fretting phenomenon can be removed by providing the plating layer on the back of the back metal layer.
According to a sixth aspect of the invention, the plating layer may be provided on the back of the back metal layer, and it is preferred that this plating layer has the same composition as that of the Sn-based overlay layer or that of the intermediate plating layer. In this constitution, it is possible to simultaneously perform both of the steps of providing the back plating layer and of providing the Sn-based overlay layer or the intermediate plating layer.